British officials said Wednesday they have charged two Russian men with the nerve-agent poisoning of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury — though they held out little hope of being able to bring the suspects to justice.

The men, who entered the country under the names Alexander Petrov and Ruslan Boshirov, were charged in absentia with conspiracy to murder, attempted murder and use of the nerve agent Novichok, the Crown Prosecution Service said.

The two men are said to have entered the country two days before the Skripals were poisoned 0:52

The U.K. is not asking Moscow to extradite the men because Russian law forbids extradition of the country's citizens, prosecutor Sue Hemming said Wednesday.

Britain has issued a European arrest warrant for the two, meaning they could be detained if they leave Russia for another European country. But assistant police commissioner Neil Basu conceded it was "very, very unlikely" police would be in a position to arrest them any time soon.

The men, both about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned on March 4, police said. Basu said the passports were genuine but the men were probably using aliases, and appealed the public "to come forward and tell us who they are."

Sergei Skripal — a former Russian agent who had been convicted in his homeland of spying for Britain — and his daughter were found collapsed on a bench in the cathedral city of Salisbury, 140 kilometres southwest of London. They spent weeks hospitalized in critical condition and are now recovering in a secret location for their own protection.

Sergei Skripal and his daughter Yulia survived nerve-agent poisoning in Britain after spending weeks in hospital. Although Moscow has denied any involvement in the attack, two Russian men have been charged. (Misha Japaridze/AP; Yulia Skripal/Facebook via AP)

British authorities and the international chemical weapons watchdog say the Skripals were exposed to Novichok, a type of military-grade nerve agent developed by the Soviet Union during the Cold War.

British officials have blamed the Russian government for the poisoning, a charge the Kremlin has denied. The poisoning ignited a diplomatic confrontation in which hundreds of envoys were expelled by both Russia and Western nations.

Ruslan Boshirov and Alexander Petrov are seen at Salisbury train station on March 3 in an image taken from CCTV. British prosecutors have charged the two Russian men with the nerve-agent poisoning of ex-spy Sergei Skripal and his daughter Yulia. (Metropolitan Police via Associated Press)

Russian Foreign Ministry spokesperson Maria Zakharova said Wednesday that Moscow has no knowledge of the two suspects, saying their names and photos "say nothing to us."

Zakharova called on Britain to co-operate with Russian law enforcement agencies on the investigation. She has criticized London for turning down Moscow's request to see the case files.

Diplomatic fallout

Police on Wednesday gave new details about what Basu called "one of the most complex investigations" the force had ever seen.

Police released a series of images of the men as they travelled through London and Salisbury between March 2 and March 4. Police say the two men took a flight back to Moscow from Heathrow Airport on the evening of March 4, hours after the Skripals were found collapsed on a park bench in Salisbury.

Police believe the nerve agent used to poison the Skripals was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and applied to the front door of Sergei Skripal's house.

Dawn Sturgess died after being exposed to the nerve agent Novichok, and her partner Charlie Rowley was hospitalized. British authorities have yet to lay charges in that incident. (Metropolitan Police via AP)

More than three months later, the bottle was found by a local man, Charlie Rowley. He was hospitalized and his girlfriend Dawn Sturgess died after being exposed to the contents.

The Organization for the Prohibition of Chemical Weapons confirmed Tuesday that Rowley and Sturgess were also exposed to Novichok.

Police are still trying to determine where the bottle was between the Skripal poisoning in March and its discovery by Rowley on June 27. As a result, Basu said, police are not yet ready to bring charges in the second poisoning.